As is well known, the conventional field effect transistor (FET) is made mainly from semiconductor materials (see FIG. 1a), where the electric carriers in the semiconductor region between source and drain are influenced by a voltage applied to the gate. In other words, the conductance between the source and drain is controlled by the gate voltage.
When the size of the transistor (L in FIG. 1a) is reduced to nanometer scale (<15 nm), the separation between the source and drain is too small to allow the gate to effective affect the carrier density. As a consequence, a nanoscale FET will not work in the same fashion as a semiconductor FET.
A prior art hybrid organic transistor has a structure as shown FIG. 1b, where the conductance of an organic layer between source and drain is controlled by the voltage applied to the gate, in the same way that the conductance between the source and drain is controlled by the gate voltage on the semiconductor transistor of FIG. 1a. By “hybrid” is meant that the channel is an organic compound, but everything else is conventional Si technology layout; see, e.g., C. R. Kagan et al, Science, Vol. 286, pp. 945–947 (Oct. 29, 1999). Usually, a relatively high gate voltage is needed to change the conductance of the organic layer. The conductance and mobility for the organic layer is low, usually smaller than 1 cm2/(V·s), the gate voltage is large (30 to 50V), and therefore the switching speed of the transistor is slow. It will be noted that in this design the current channel is organic, and the gate oxide and the gate are those used in conventional silicon technology. In contrast, in the embodiments disclosed herein, the organic layer is the insulator with polarization, which is controlled by the applied external field. The gating effect on a semiconducting channel is achieved by the electrostatic potential created by an organic ferroelectric insulator, which may be in direct contact with the channel, or separated from it by a dielectric layer.
A prior art ferroelectric transistor has a structure as shown in FIG. 1c, where the polarization of the underlying ferroelectric layer influences the conductance of the underneath the semiconductor layer between the source and drain. Such a transistor can be used for nonvolatile memory applications, and is usually called a ferroelectric memory field effect transistor (FEMFET). The problem with making short gate devices (L<100 nm) is that the switching properties of the ferroelectric element apparently deteriorate at smaller sizes.
A prior art chemical FET (chemFET) sensor is illustrated in FIG. 1d, and is used for detecting particular chemical species. The chemFET comprises the channel region and source and drain electrodes, all grown on an insulator layer. The channel is exposed to the molecules in the surrounding environment, which can chemisorp directly on a channel region. The chemisorption changes the density of carriers in the channel and, consequently, its conductance. The change in conductance is used to detect the molecular species (hence, the term “chemFET”). ChemFET devices are described by, for example, A. Barbaro et al, Advanced Materials, Vol. 4, pp. 402–408 (1992).
There is a significant technological opportunity for a transistor and/or sensor element that is based on molecular materials for its operation.